Proximity sensors for detecting an actual or relative distance between the sensor and an object are known in the art. For example, U.S. Pat. No. 6,621,278 to Arie Ariav discloses a method of measuring a distance by transmitting a cyclically-repeating wave. The wave is then received at a second location in the medium. The system detects a predetermined point in the cyclically-repeating wave that is received at the second location and continuously changes the frequency of transmission of the cyclically-repeating energy wave in accordance with the detected point of each received cyclically-repeating wave received at the second location such that the number of waves received at the second location is a whole integer. The change in frequency to produce a measurement of the predetermined parameter is used to determine the distance the wave has traveled. However, this system has drawbacks, particularly in that the sensor is unduly complex both in electronic implementation and in sensor construction.
Other types of detectors, primarily for detecting the presence or absence of an object, use ultrasonic and radio frequency transmitters and detectors that receive reflected energy when an object is present in an area of interest. These detectors however cannot be used practically to detect a relative or actual distance, particularly in very short distances. In certain settings, the amount of RF energy generated by these types of device is unacceptable due to interference. Moreover, some people have concerns about constant exposure to RF energy.
Many applications require low power consumption and detection of a relative distance within a range of interest. For example, cushions for wheelchairs must be inflated to a pressure that is sufficient to properly immerse the person in the cushion to prevent the formation of decubitus ulcers on the person in the wheelchair. However, often the people bound to the wheelchair do not have the ability to feel when they are properly immersed in the cushion, such as a paraplegic or quadriplegic person. For those people, others must periodically check the person's immersion within the cushion to ensure the person is not in an overinflated state, such that only a small portion of the person's body is bearing their weight, or in an underinflated state, such that the person has “bottomed out” and is no longer supported entirely by the cushion. Similarly in a cushion not inflated with air, problems also exist when determining the proper cushion immersion. However, presently, no acceptable means of detecting the immersion of a person in a cushion exists. Only indirect measurement of pressure internally in the cushion is available. This type of measurement is dependant upon the materials of construction and structural conformation all creating significant limitations in the applicability of the measurement.
Likewise, people bound to hospital beds must avoid decubitus ulcers when confined to the bed for long periods of time. To accomplish this, inflation mattresses are commonly used, and the inflation level of the mattress must be monitored in order to maintain the proper inflation level to prevent overinflation or underinflation of the mattress. Moreover, because the person's weight is concentrated over their entire back side, multiple locations must be checked for underinflation or overinflation. As a result, a sensor which is divided into zones to check the immersion of the patient within the mattress is needed.